Current treatments for breast cancer focus on the killing malignant cells but there is a substantial population of patients for whom these approaches ultimately fail. The long-term objective of this work is to develop an adjuvant therapy that targets the tumor-promoting stroma of cancer as a way to potentiate current approaches. Tumor-associated macrophages (TAMs) are activated in the tumor microenvironment and facilitate disease progression by promoting angiogenesis and tumor cell growth, and by suppressing the adaptive immune response. We have recently identified a peptide that preferentially recognizes murine tumor-associated macrophage but not resting macrophage, other leukocytes, or tumor cells. In this work, we will prepare multivalent targeting constructs to deliver pro-apoptotic agents to TAMs in a mouse model of mammary cancer. In addition, we will develop analogous targeting peptides for human TAMs. These technologies will be combined in a new construct for targeted ablation of human TAMs. Our goal is to move this technology toward clinical development and to provide an adjuvant treatment that is effective even for women with hormone-unresponsive or triple negative breast cancer.